The invention relates to a process for producing a casting core, which is used for forming within a casting a cavity intended for cooling purposes, through which a cooling medium can be conducted.
Casting cores are mold parts which are provided in a casting mold and displace the solidifiable material poured into the casting mold, and in this way form cavities in the cast end product, or casting. The production of heat-exposed products obtained as castings with the aid of the casting cores referred to above is of particular interest. Such end products are, for example, turbine blades which are exposed to very high temperatures in a gas turbine installation. To increase the service life of the turbine blades, they are provided in a known way with inner cooling channels, through which a cooling medium, preferably cooling air or water vapor, is passed for cooling purposes. Through cooling by means of constant heat removal, the material of the turbine blades is not heated up to the temperatures actually prevailing in its surroundings, thereby allowing the material to be preserved and its service life to be considerably prolonged.
In the area of the combustion chamber provided in a gas turbine installation it is also necessary for reasons relating to material-preserving aspects to cool the combustion chamber walls in a specifically selective manner and to provide them in a corresponding way with cooling channels.
In the cooling of turbine blades and in the cooling of combustion chamber walls there is the problem that the heat flow directed from outside onto the components is to be removed as efficiently as possible through the cooling fluid flowing through in the cooling channels. For this reason, the wall surfaces of the cooling channels taking part in the heat transfer should have internal heat transfer coefficients which are as high as possible. Various excitation mechanisms, such as the provision of ribs or pins for example, are used in most cases for this purpose, to increase the local surface area via which the heat flow is removed to the cooling fluid.
Furthermore, it is generally known that rough surfaces produce a greater heat transfer than smooth services. This effect is particularly dependent on the ratio of the height of the roughness to the hydraulic diameter of the cooling channel and on the ratio of the height of the local roughness to the thickness of the laminar underlayer of the flow and temperature boundary layer which forms when a cooling fluid flows through a cooling channel. However, roughness elevations on the surface of a cooling channel only have an influence on an increase in the heat transfer if they are of a height which rises above the laminar underlayer.
A great advantage of rough surfaces with regard to a desired increase in the heat transfer of a heated component to a cooling medium in comparison with the above known measures of using ribs and pins or similar heat-transfer-increasing internal components is essentially the much lower pressure loss which occurs when the cooling medium flows through a xe2x80x9croughenedxe2x80x9d cooling channel.
This relationship is to be explained in more detail with reference to FIG. 1. Plotted on the y axis of the diagram represented in FIG. 1 is the resistance coefficient f which a flow has when flowing through a flow channel, as a function of the Reynolds"" numbers Re plotted on the x axis of the diagram. The graphs a to e entered in the diagram represent flow situations for different types of ribs in which a flow through a flow channel provided with lines of ribs. The solid line corresponds to the flow case of a through-flow channel with a smooth surface. The dashed line plotted directly above the solid line represents a flow case in which the through-flow channel has a roughened surface, with a roughness ratio R/ks of 60. R signifies here the hydraulic radius of the flow channel and ks corresponds to the magnitude of the equivalent sand grain roughness of the surface. For a through-flow channel with a diameter of 10 mm, the R/ks ratio of 60 corresponds to a roughness elevation of about 80 xcexcm. It can be clearly seen from the function profiles entered in the diagram of FIG. 1 that the rough wall has an only approximately 50% higher resistance, and consequently pressure loss, than a smooth through-flow channel, but has a considerably lower pressure loss or resistance than the ribbed flow channels of cases a to e.
A further aspect in favor of the use of roughened surfaces in cooling channels can be explained with reference to FIG. 2, which shows a diagram which represents the xe2x80x9cthermal performancexe2x80x9d of turbulators, such as ribs for example, as opposed to a roughened surface. The values plotted on the y axis of the diagram in FIG. 2
G=(St/St0)/(f/f0)⅓
which shows the relative increase in heat transfer for an equal pumping capacity in the system. These values consequently indicate the xe2x80x9cthermal performancexe2x80x9d of the system (of the ribs) and consequently their relative efficiency in comparison with a smooth channel. A value of G=1 in this case corresponds to a smooth channel.
Plotted on the x axis of the diagram in ascending sequence are Reynolds"" numbers Re of the cooling medium within the flow channel. The function profiles a to e represent the efficiency of various rib arrangements within the flow channel, assuming that a constant pumping capacity is available. The steady decrease in efficiency G for various rib configurations with increasing Reynolds"" numbers can be clearly seen. The dashed line, on the other hand, represents the case of a roughened surface within a cooling channel, which by contrast with the above function profiles has a curve which rises with increasing Reynolds"" numbers. Even at Reynolds"" numbers of approximately 100,000, the rough wall has better heat transfer properties than two known different types of ribs. If the function profile of the rough surface is extrapolated to even higher Reynolds"" numbers, as are encountered in combustion chamber cooling systems for example, the rough surface inside cooling channels is the best solution if the object is to obtain the maximum increase in heat transfer for a given pumping capacity.
This is particularly true because, for very high Reynolds"" numbers, only the conditions in the direct vicinity of the wall remain significant and the applied form of the turbulators merely blocks the outer flow and consequently increases the pressure loss, but no longer contributes to intensifying the heat transfer at the wall.
On the basis of the comments made above on the significance of a specifically selective incorporation of surface roughnesses in cooling channels, possibilities for producing specifically selective surface roughnesses, in cooling channels in particular, are to be specified.
The mold parts provided with cooling channels are preferably produced by means of casting processes and serve, for example, as subassemblies of gas turbine installations to be subjected to heat. The cooling channels within a turbine blade for example, can be very filigree and can be accessed from outside only with difficulty, or not at all, for local finishing after completion of the turbine blade. Ways by which a desired surface roughness can be obtained with a surface finish which has to conform to certain roughness values must be found. Since the end products concerned are produced within a casting process, ways of obtaining the desired surface roughness before or during the casting process, or while the cast end product is cooling down, must be found.
Accordingly, one object of the invention is to provide measures by which a desired surface roughness on the end product can be produced during the casting operation. In particular, inaccessible cavities within the end product, which are preferably designed as cooling channels, are to have a desired surface roughness without any finishing steps.
One solution for achieving the object on which the invention is based includes a process for producing a casting core for forming a casting having a cavity intended for cooling purposes through which a cooling medium can be conducted, the process comprising: providing the casting core with surface regions in which there is incorporated in a specifically selective manner a surface roughness which transfers itself during the casting operation to surface regions of the casting enclosing the cavity and leads to an increase in heat transfer between the cooling medium and the casting.
The invention is based on the idea of covering the casting cores which are to be provided for the casting operation, to produce cavities within the end product to be produced, with an artificial roughness which transfers itself during the casting operation to the surface of the end product to be produced, preferably to those surface regions which enclose a cavity which forms a cooling channel in the completed casting.
It has been perceived as a preference that the casting core intended for forming a cavity within the end product can be roughened by prior working of its surface. The degree of roughness transferred to the surface of the casting core can be applied, for example, by means of a core tool. For this purpose, the surface of the core tool is roughened to a desired extent by means of spark erosion. The degree of roughness to be applied to the core tool can be specifically set by the voltage to be applied to the spark electrode and/or by choosing the distance between the spark electrode and the core tool to be roughened.
The surface roughness applied in this way to the surface of the core tool transfers itself during the production process for the casting core to the casting core surface and subsequently during the casting process and the following cooling down of the end product to the corresponding inner surface contour of the end product.
Casting cores are usually produced from a figuline mass which has to be fired for hardening. Before firing, shaped casting cores are referred to as xe2x80x9cgreen coresxe2x80x9d and, to incorporate a surface roughness in this state or in the fired state, can be roughened by means of sand blasting or selective further roughening techniques, such as grinding and abrading operations for example.
Similarly, the casting core may be roughened as a green core with the aid of a cold or heated tool which has a defined roughness structure, by pressing into the surface of the casting core in the customary way.
Further roughening techniques which lead to a specifically selective surface roughness on the casting core are of course also conceivable; what is important is that a defined roughness is provided on the casting core in such a form as to allow a specifically set surface roughness to be produced in the end product, for example in the cooling channel of a turbine blade or in the cooling channel of a combustion chamber wall.
The surface roughness is to be set in such a way that it is adapted to the following flow conditions which prevail inside the cooling channel and to the desired heat transfer coefficient.
In principle, the following relationship between the resistance coefficient f and the heat transfer coefficient xcex1, or the Stanton number St, applies:       α          α      0        =            St              St        0              =                  (                  f                      f            0                          )            0.63      
In the above equation, the variables denoted by the index zero represent reference variables of a smooth channel, while the variables without an index apply to a rough channel. In the event that the ratio f/f0 is  greater than 4, this ratio is to be equated with 4.
After determining the desired increase in the heat transfer coefficient, the associated roughness variable R/ks can be read off from the diagram representation according to FIG. 3 (taken as a basis below) and used for producing the core tool. The maximum achievable increase in heat transfer in this case is, however, St/St0=2.4. There is consequently no point in using roughnesses which make the resistance coefficient more than 4 times as great as in the case of a smooth channel wall surface.